The invention relates to a device for driving a preheating roll assembly in a unit for producing films and to an associated method for the operation thereof.
When producing plastics films the screen film issuing from an extruder is usually first of all cooled via a chilling roll rotating in a water bath. Using what is known as a preheating roll assembly the film is again heated to a temperature which allows subsequent stretching of the film.
Heating of the film, which is desired in terms of process-engineering, by preheating rolls leads to a change in length which in turn requires speed graduation of the preheating rolls. According to the prior art this is achieved in that the individual rolls are driven by a common motor via belts. In this connection, each roll comprises a belt pulley over which a belt runs that is driven by the common motor. Speed graduation of the rolls is attained by appropriate graduation of the diameters of the belt pulleys. One drawback in this connection is that the graduation, once selected, corresponds to only one heating profile. A different graduation of the belt pulleys is required in the case of other process parameters since otherwise excessive stresses in the film (lead too high) or a slackening of the film (delay) would occur. To change the graduation of the belt pulleys they have to be replaced, and this is linked to very high expenditure.
To solve this problem an individual drive could be associated with each preheating roll. While the applicable rotational speed can be correctly adjusted in any operating case on the individual rolls hereby, a separate motor with sensor and converter have to be provided for each preheating roll for this purpose. The adjustment of the individual motors for each operating case is also very laborious.
Document DE 30 26 129 A1 describes a metal strip stretching unit with a plurality of pulling and retaining rolls, it being possible for some of the rolls to be decoupled from a common main drive motor.